1. Field of the Invention
The present invention relates to a flow sensor for measuring the flow velocity or flow rate of a fluid such as intake air for an internal combustion engine. More specifically, it relates to a flow sensor, equipped with a heating element, for measuring the flow rate of a fluid based on a heat transfer phenomenon from the heating element or a part heated by the heating element to the fluid.
2. Description of the Prior Art
FIGS. 13(a) and 13(b) are diagrams showing the constitution of a flow detection element (diaphragm sensor) 51 used in a conventional flow sensor disclosed by Japanese Laid-open Patent Application No. 4-230808, for example. FIG. 13(a) is a plan view and FIG. 13(b) is a sectional view cut on line Dxe2x80x94D of FIG. 13(a). In FIGS. 13(a) and 13(b), reference numeral 1 denotes a plate substrate made from a silicon semiconductor. A cavity 12 which has a trapezoidal section and does not communicate with the front side of the plate substrate 1 is formed in a center portion of the rear side of the plate substrate 1 by anisotropic etching to fabricate a thin diaphragm 13 in the plate substrate 1 on the bottom side of the cavity 12, that is, the front side of the plate substrate 1.
A thin film heating element 3 is formed at a center portion of the surface of the diaphragm 13 and thin film resistance thermometers 52 and 53 are formed symmetrical on both sides of the heating element 3 at a predetermined interval therebetween. Slit portions 54a and 54b which are belt-like holes and extend through the diaphragm 13 are formed between the heating element 3 and the resistance thermometers 52 and 53 in a longitudinal direction, and slit portions 55a and 55b which consist of a plurality of square holes extending through the diaphragm 13 and aligned with one another are formed outside the resistance thermometers 52 and 53 in a longitudinal direction. Slit portions 56c, 56d, 57c and 57d which are holes extending through the diaphragm 13 are formed at both ends in a longitudinal direction of the heating element 3 and the resistance thermometers 52 and 53, respectively. These slit portions 54a to 57d are formed by general photolithography or wet or dry etching.
In the above FIGS. 13(a) and 13(b), the electrodes of the heating element 3 and the resistance thermometers 52 and 53 and thin-film conductor patterns forming the power lines of the heating element 3 and the resistance thermometers 52 and 53 formed on the plate substrate 1 are omitted.
A description is subsequently given of the operation of the above flow detection element 51 of the prior art.
The front side (heating element 3 side) of the flow detection element 51 is made parallel to the flow passage of a fluid to be measured, the longitudinal directions of the heating element 3 and the resistance thermometers 52 and 53 are made perpendicular to the flow of the fluid, and a current to be applied to the heating element 3 is controlled such that the temperature of the heating element 3 should be higher than the temperature of the fluid by a predetermined value. Since the resistance thermometers 52 and 53 are arranged symmetrical about the heating element 3, when the fluid does not flow (flow velocity is zero), the temperatures of the above resistance thermometers 52 and 53 are equal to each other.
When the fluid flows in a direction shown by an arrow V, the resistance thermometer 52 on an upperstream side is cooled and the temperature thereof becomes lower than that when the flow velocity is zero. A reduction in the temperature of the above resistance thermometer 52 becomes greater as the flow velocity increases. Meanwhile, since the resistance thermometer 53 on a downstream side is located on the downstream side of the heating element 3, when the flow velocity is the same, the temperature of the resistance thermometer 53 does not become as low as that of the resistance thermometer 52 on the upperstream side. Therefore, there is a temperature difference between the resistance thermometer 52 on the upperstream side and the resistance thermometer 53 on the downstream side according to the flow velocity of the fluid. Then, by detecting a resistance difference between the resistance thermometer 52 and the resistance thermometer 53, which corresponds to the above temperature difference, by means of detection means such as an unshown Wheatstone bridge circuit incorporating the resistance thermometers 52 and 53, the flow velocity of the fluid can be measured.
Thus, in the above prior art, changes in output caused by the adhesion of dust are reduced by forming the cavity 12 in the rear side of the plate substrate 1 to fabricate the thin diaphragm 13. Further, the slit portions 54a to 57d are formed in the diaphragm 13 to reduce a heat flow from the heating element 3 to the resistance thermometers 52 and 53, thereby suppressing a rise in the temperatures of the resistance thermometers 52 and 53 and reducing a heat flow from the heating element 3 to the plate substrate 1 to improve sensitivity.
To obtain high sensitivity and responsibility for a flow detection element having such a diaphragm structure, the heat responsibility of the diaphragm 13 must be improved by reducing the thickness of the diaphragm 13 regardless of the existence of the slit portions. However, when the thickness of the diaphragm 13 is reduced, the ratio of the thickness of the diaphragm 13 to the thickness of a heat sensitive resistor film forming the heating element 3 and the resistance thermometers 52 and 53 becomes large. Therefore, as the thickness of the diaphragm 13 decreases, the difference of a material structure in a thickness direction between a portion with the heat sensitive resistor film and a portion without the heat sensitive resistor film becomes larger, whereby the diaphragm 13 deforms (initial deformation) when the heat sensitive resistor film and the cavity are formed. This deformation is caused by the difference of internal stress between the materials of the films. When the initial deformation of the diaphragm 13 occurs and electricity is applied to the heating element 3 to generate heat, the deformation of the diaphragm 13 becomes larger due to the differences of thermal or mechanical properties between the material of the diaphragm 13 (silicon which is the material of the substrate) and the material of the heat sensitive resistor film such as the heating element 3 formed thereon (for example, a metal material such as platinum). When the deformation is large, large stress is generated between the diaphragm 13 and the heat sensitive resistor film, thereby causing the heat sensitive resistor film forming the heating element 3 and the resistance thermometers 52 and 53 to be separated from the surface of the diaphragm 13. This exerts an adverse effect on the detection characteristics of the flow sensor.
Further, when the large deformation of the diaphragm 13 occurs, there are differences in the amount of deformation of the film when it serves as a flow sensor due to differences in the thermal or mechanical properties of the film, which may influence the detection characteristics of the flow meter and make accurate flow detection impossible.
If the above deformation is asymmetrical within the plane of the diaphragm 13 at the time of forming thin-film patterns or applying electricity for heating, the separation of the film and the difference of deformation become more marked, thereby deteriorating the detection characteristics of the flow sensor.
In view of the above problems of the prior art, it is an object of the present invention to provide a flow sensor which has excellent responsibility, sensitivity and reliability and high flow detection accuracy by suppressing the deformation of a diaphragm.
According to a first aspect of the present invention, there is provided a flow sensor in which additional patterns are formed on a diaphragm or a diaphragm and a portion therearound so that thin film patterns formed on the surface of the diaphragm formed in a plate substrate of a flow detection element become almost symmetrical on the diaphragm.
According to a second aspect of the present invention, there is provided a flow sensor in which the additional patterns are formed at positions where they are almost symmetrical to connection patterns on the diaphragm so that the thin film patterns formed on the diaphragm become almost symmetrical on the diaphragm.
The additional patterns of the second aspect of the present invention will be referred to as xe2x80x9cfirst additional patternsxe2x80x9d hereinafter.
According to a third aspect of the present invention, there is provided a flow sensor in which the first additional patterns are dummy patterns which do not contribute to power supply to a heating element.
According to a fourth aspect of the present invention, there is provided a flow sensor in which the dummy patterns (first additional patterns) are connected to the pattern of the heating element.
According to a fifth aspect of the present invention, there is provided a flow sensor in which the dummy patterns (first additional patterns) are not connected to the pattern of the heating element but to the ground of a flow detection circuit or the shielding member of the flow sensor.
According to a sixth aspect of the present invention, there is provided a flowsensor in which the connection patterns and the first additional patterns are formed on lines connecting the corner portions of the pattern of the heating element and the corner portions of the diaphragm, respectively.
According to a seventh aspect of the present invention, there is provided a flow sensor in which the connection patterns are formed to surround at least part of the pattern of the heating element.
According to an eighth aspect of the present invention, there is provided a flow sensor in which the total area of the connection patterns and the first additional patterns is half or more the area of the diaphragm excluding the pattern of the heating element.
According to a ninth aspect of the present invention, there is provided a flow sensor in which the connection patterns are combined with the first additional patterns.
According to a tenth aspect of the present invention, there is provided a flow sensor in which the width of the first additional patterns is larger than the width of the pattern of the heating element and the first additional patterns form part of a current path for the heating element.
According to an eleventh aspect of the present invention, there is provided a flow sensor in which the pattern of the heating element, the connection patterns and the first additional patterns are formed of the same metal layer.
According to a twelfth aspect of the present invention, there is provided a flow sensor in which the thickness of the pattern of the heating element, the thickness of the connection patterns and the thickness of the first additional patterns are ⅕ or less the thickness of the diaphragm.
According to a thirteenth aspect of the present invention, there is provided a flow sensor in which the additional patterns are dummy patterns formed in an area other than the power lines of the diaphragm and formed at positions where they are almost symmetrical on the diaphragm.
The additional patterns of the thirteenth aspect of the present invention will be referred to as xe2x80x9csecond additional patternsxe2x80x9d hereinafter.
According to a fourteenth aspect of the present invention, there is provided a flow sensor in which third additional patterns formed at the periphery of the diaphragm and connection patterns for connecting the third patterns to the dummy patterns (second additional patterns) are formed, and the total of the widths of the connection patterns at the boundary of the diaphragm is half or less the whole circumference of the boundary of the diaphragm.
According to a fifteenth aspect of the present invention, there is provided a flow sensor in which the third additional patterns are not connected to the pattern of the heating element but to the ground of a flow detection circuit or the shielding member of the flow sensor.
According to a sixteenth aspect of the present invention, there is provided a flow sensor in which the connection patterns are laid over the corner portions of the diaphragm.
According to a seventeenth aspect of the present invention, there is provided a flow sensor in which the dummy patterns (second additional patterns) are formed to surround at least part of the pattern of the heating element.
According to an eighteenth aspect of the present invention, there is provided a flow sensor in which the total area of the dummy patterns (second additional patterns) on the diaphragm is half or more the area of the diaphragm excluding the pattern of the heating element.
According to a nineteenth aspect of the present invention, there is provided a flow sensor in which the pattern of the heating element and the dummy patterns (second additional patterns) are formed of the same metal layer.
According to a twentieth aspect of the present invention, there is provided a flow sensor in which the thickness of the pattern of the heating element and the thickness of the dummy patterns (second additional patterns) are ⅕ or less the thickness of the diaphragm.
The above and other objects, features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.